Allow subclassing of AttrsInstance and Protocol at once (#1172)

.github/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
           python -Im coverage html --skip-covered --skip-empty
 
           # Report and write to summary.
-          python -Im coverage report | sed 's/^/    /' >> $GITHUB_STEP_SUMMARY
+          python -Im coverage report --format=markdown >> $GITHUB_STEP_SUMMARY
 
           # Report again and fail if under 100%.
           python -Im coverage report --fail-under=100

.gitignore

@@ -19,3 +19,4 @@ tmp*
 attrs.docset
 attrs.tgz
 Justfile
+t.py

changelog.d/1172.change.md

@@ -0,0 +1,2 @@
+`attrs.AttrsInstance` is now a `typing.Protocol` in both type hints and code.
+This allows you to subclass it along with another `Protocol`.

pyproject.toml

@@ -133,6 +133,9 @@ exclude_lines = [
     "pragma: no cover",
     # PyPy is unacceptably slow under coverage.
     "if PYPY:",
+    # not meant to be executed
+    ': \.\.\.$',
+    '^ +\.\.\.$',
 ]
 
 

src/attr/__init__.py

@@ -9,6 +9,7 @@ from typing import Callable
 
 from . import converters, exceptions, filters, setters, validators
 from ._cmp import cmp_using
+from ._compat import Protocol
 from ._config import get_run_validators, set_run_validators
 from ._funcs import asdict, assoc, astuple, evolve, has, resolve_types
 from ._make import (
@@ -31,7 +32,7 @@ ib = attr = attrib
 dataclass = partial(attrs, auto_attribs=True)  # happy Easter ;)
 
 
-class AttrsInstance:
+class AttrsInstance(Protocol):
     pass
 
 

src/attr/_compat.py

@@ -18,6 +18,15 @@ PY310 = sys.version_info[:2] >= (3, 10)
 PY_3_12_PLUS = sys.version_info[:2] >= (3, 12)
 
 
+if sys.version_info < (3, 8):
+    try:
+        from typing_extensions import Protocol
+    except ImportError:  # pragma: no cover
+        Protocol = object
+else:
+    from typing import Protocol  # noqa: F401
+
+
 def just_warn(*args, **kw):
     warnings.warn(
         "Running interpreter doesn't sufficiently support code object "

tests/test_compat.py

@@ -4,6 +4,8 @@ import types
 
 import pytest
 
+import attr
+
 
 @pytest.fixture(name="mp")
 def _mp():
@@ -50,3 +52,13 @@ class TestMetadataProxy:
 
         with pytest.raises(AttributeError, match="no attribute 'setdefault'"):
             mp.setdefault("x")
+
+
+def test_attrsinstance_subclass_protocol():
+    """
+    It's possible to subclass AttrsInstance and Protocol at once.
+    """
+
+    class Foo(attr.AttrsInstance, attr._compat.Protocol):
+        def attribute(self) -> int:
+            ...